Wheat is an important crop as a food staple and nutritional agent, and has been domesticated for about 10,000 years. In 2007, world production of wheat was 607 million tons, which makes wheat the third most-produced cereal after maize and rice. Wheat grain is a staple food used to make flour for leavened, flat, and steamed breads, biscuits, cookies, cakes, breakfast cereal, pasta, noodles, couscous, and for fermentation to make beer, alcohol, vodka, or biofuels. Wheat is also planted to a limited extent as a feed and/or forage crop for livestock and as a construction material for roofing thatch.
Wheat is divided into five main market classes, which includes the common wheat (Triticum aestivum L.) classes: hard red winter, hard red spring, soft red winter, soft and hard white, and durum (Triticum turgidum L.). Common wheats are used in numerous food products, such as bread, cookies, cakes, crackers, and noodles. In general, the hard wheat classes are milled into flour used for breads and the soft wheat classes are milled into flour used in products, such as pastries, crackers, breakfast cereals, and soup thickeners. Wheat starch can be used in the food and paper industries, as laundry starches, and in other products.
The differences between red and white wheat are not very pronounced and are based on the bran color, which contribute to the identification of wheat color. Wheat can be either red or white, with white wheat lacking the pigment that gives red wheat its color. The absence of bran color in white wheat results in a flour that is slightly sweeter. The greatest difference between grains is actually between soft and hard wheat. The varying protein content levels, while not nutritionally significant, result in very different uses in terms of baking.
In order to fulfill their demands, flour millers must choose among available wheat cultivars grown in different regions, depending upon soil and climate characteristics, and having different milling properties. For example, soft red winter wheats are typically grown in Ohio, Indiana, and areas of the Southeastern U.S. Meanwhile, soft white wheats are generally grown in the Pacific Northwest and Michigan. Hard red winter wheats are primarily grown in Kansas, Nebraska, Oklahoma, and Texas. Hard wheats typically have higher gluten strength properties that are better suited for bread baking than soft wheats. Therefore, commercial bread bakers are generally biased in favor of flours made primarily from hard wheat cultivars, and these cultivars are demanded by millers accordingly.
Currently, red wheat is more readily available in the United States than white wheat. Hard white wheat in the United States was produced on less than 2 million acres in 2006. Hard red wheats are characterized by a relatively strong wheat flavor that consumers may not want for whole wheat bread products. Red wheat also has a distinctive bitter taste due to the tannins and phenolic compounds in the bran that many consumers find unpleasant, and which is offset in the final baking product by the presence of expensive sweeteners. Moreover, red wheats will have a red color in the intact wheat kernel and its outer layers. The distinct red hue of whole wheat flour milled from hard red wheat cultivars may be problematic for bread products like whole wheat croissants and Danish rolls that consumers typically associate with a white hue. Furthermore, bran separated from hard red wheat cultivars is generally only suitable for animal feeds, and therefore is less valuable to the miller than brans derived from white wheat cultivars that may be used in breakfast cereals and other bran products consumed by humans. Red wheat also may have lower milling performance compared to white wheat, because a significantly higher extraction rate may be used with white wheat without sacrificing flour color.
Wheat breeders continually develop stable, high yielding wheat cultivars that are agronomically sound and have good grain quality for its intended use. To accomplish this goal, the wheat breeder must select and develop wheat plants that have the traits that result in superior cultivars. These selection processes, which ultimately lead to the marketing and distribution of the wheat cultivar, can take many years from the time the first cross is made. Development of new wheat cultivars is therefore a time-consuming process that requires precise forward planning, efficient use of resources, and a minimum of changes in direction.
Choice of breeding or selection methods depends on the mode of plant reproduction, the heritability of the trait(s) being improved, and the type of cultivar used commercially; e.g., F1 hybrid cultivar, pure-line cultivar, etc. For highly heritable traits, a choice of superior individual plants evaluated at a single location can be effective, whereas for traits with low heritability, selection may be based on mean values obtained from replicated evaluations of families of related plants. Popular selection methods commonly include pedigree selection, modified pedigree selection, mass selection, and recurrent selection.
Pedigree breeding can be used for the improvement of self-pollinating crops. Two parents that possess favorable, complementary traits are crossed to produce an F1. An F2 population is produced by selfing or sibbing one or several F1 so selection of the best individuals may begin in the F2 population; then, beginning in the F3, the best individuals in the best families are selected. Replicated testing of families can begin in the F4 generation to improve the effectiveness of selection for traits with low heritability. At an advanced stage of inbreeding (i.e., F5, F6 and F7), the best lines or mixtures of phenotypically similar lines are tested for potential release as new cultivars.
Backcross breeding is used to transfer genes for simply inherited, qualitative traits from a donor parent into a desirable homozygous cultivar that is utilized as the recurrent parent. The source of the traits to be transferred is called the donor parent. After the initial cross, individuals possessing the desired trait or traits of the donor parent are selected and then repeatedly crossed (backcrossed) to the recurrent parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) plus the desirable trait or traits transferred from the donor parent. This approach has been used extensively for breeding disease resistant varieties.
Another breeding method that can be utilized is single-seed descent. This procedure in the strict sense refers to planting a segregating population, harvesting a sample of one seed per plant, and using the one-seed sample to plant the next generation. When the population has been advanced from the F2 to the desired level of inbreeding, the plants from which lines are derived will each trace to different F2 individuals. The number of plants in a population declines each generation due to failure of some seeds to germinate or some plants to produce at least one seed. As a result, not all of the F2 plants originally sampled in the population will be represented by a progeny when generation advance is completed. In a multiple-seed procedure, wheat breeders commonly harvest one or more spikes (heads) from each plant in a population and thresh them together to form a bulk. Part of the bulk is used to plant the next generation and part is put in reserve. The procedure has been referred to as modified single-seed descent. The multiple-seed procedure has been used to save labor at harvest. It is considerably faster to thresh spikes with a machine than to remove one seed from each by hand for the single-seed procedure. The multiple-seed procedure also makes it possible to plant the same number of seeds of a population each generation of inbreeding. Enough seeds are harvested to make up for those plants that did not germinate or produce seed.
Bulk breeding can also be used. In the bulk breeding method an F2 population is grown. The seed from the populations is harvested in bulk and a sample of the seed is used to make a planting the next season. This cycle can be repeated several times. In general when individual plants are expected to have a high degree of homozygosity, individual plants are selected, tested, and increased for possible use as a variety.
The production of doubled haploids can also be used for the development of homozygous lines in the breeding program. Doubled haploids are produced by the doubling of a set of chromosomes (1N) from a heterozygous plant to produce a completely homozygous individual. This can be advantageous because the process omits the generations of selfing needed to obtain a homozygous plant from a heterozygous source. Various methodologies of making doubled haploid plants in wheat have been developed.
Although most commercial wheat production is from pure-line inbred cultivars, hybrid wheat is also grown. Hybrid wheat is produced with the help of cytoplasmic male sterility, nuclear genetic male sterility, or chemicals. Various combinations of these three male-sterility systems have been used in the production of hybrid wheat.